Changes 5.5.40:
InnoDB: An ALTER TABLE ... ADD FOREIGN KEY operation could cause a serious error.
InnoDB: With a transaction isolation level less than or equal to READ COMMITTED, gap locks were not taken when scanning a unique secondary index to check for duplicates. As a result, duplicate check logic failed allowing duplicate key values in the unique secondary index.
InnoDB: During recovery, a segmentation fault would occur when marking a table as corrupt.
yaSSL client code did not validate the encryption size or session ID length, which could cause the client to exit.
MySQL installation from RPM packages could fail if Postfix had been installed using yum.
yaSSL could fail preauthorization if the client supplied inaccurate buffer lengths.
Competition between threads could lead to timeout failure trying to rotate the audit log file.
On Linux (OEL6), if Sun DTrace was installed, the MySQL build failed.
LPAD() and RPAD() could cause a server exit if the pad string argument was not well formed.
The optimizer could create a zero-length column for a temporary table, causing a server exit.

(adam)